When incoming light beam enters dielectric reflective mirror with certain incident angle, there is phase shift between the p-polarization and s-polarization. In terms of Fresnel formulas and boundary conditions of electromagnetic field, the phase shift between p- and s-polarization was deduced with methods of matrix. Based on optimizing design, when light beam enters with 54 degree incident angle, phase shift of 270°±1° in the range of 1285～1345 nm was reached, at the same time, the reflectivity of dielectric mirror maintained is over 99.5%. The phase retarder was prepared with dual ion beam sputter system and then annealed at different temperatures in atmosphere, the spectrum and the phase of the sample were gained by spectrometer and the Fourier transform infrared spectroscopic ellipsometer, respectively. The results revealed that the as-deposited samples gained phase shift of 262.4°±1.8° and the reflectivity reaches over 99.6%. The error mainly originates from non-homogeneous transition layer, posed by working properties of ion source, and surface transition layer, resulting from surface adsorption for moisture and dust. The physical thickness and variety of refractive index for transition layer were gained from error analysis, and it was found that the error of the outermost layer and deviation of refractive index and designed values play a principal roles in decrease of phase shift.